Should she wear hijab in front of her Christian maternal uncle?

Your maternal uncle is considered to be a mahram for you, and on this basis it is permissible for you to take off your hijab in front of him. There is no report that the Prophet (peace and blessings of Allah be upon him) ordered the Muslim women to observe hijaab in front of their kaafir relatives. 

But the scholars have mentioned that the relative in front of whom the woman takes off her hijab should be trustworthy. This condition applies to both Muslims and kaafirs. 

They mentioned that in the context of women shaking hands with or kissing their relatives. But if her mahram is not trustworthy in the sense that he may describe her to others or be tempted by seeing her, then she should observe hijab in front of him, regardless of whether he is a Muslim or a kaafir. One of the unique opinions [mufridaat] of Imam Ahmad (may Allah have mercy on him) is that he stated that the mahram who accompanies a Muslim woman when travelling must be a Muslim, but some of his companions did not agree with him on this condition. The reason why he did not allow a kaafir to be a mahram for travel purposes is that he is not trustworthy, especially if he is a Magian. He stated that such a person could not be a mahram for his mother because he thinks that it is permissible to have intercourse with her! One of his companions stated that a Jew or a Christian might sell his mother or kill her! If we look at these reasons we will see that they could be applied to some evildoers among the Muslims, so we can see that this is not a strong argument for the idea that a kaafir cannot be counted as a mahram because of his being a kaafir. But other considerations remain valid, such as his being trustworthy or otherwise.  

This has to with mahrams. In the case of non-mahrams who are kaafirs, there was a difference of opinion among the scholars as to whether it is permissible for a kaafir woman to see a Muslim woman. The most correct opinion is that it is permissible, and the prohibition applies in cases where one cannot be sure that the woman will not describe the Muslim woman to others who are non-mahrams to her, whether that woman is a Muslim or a kaafir. 

Shaykh Ibn ‘Uthyameen (may Allah have mercy on him) was asked: 

Is it permissible for a Muslim woman to uncover her hair in front of a non-Muslim woman, especially if she will describe the Muslim woman to men among her relatives who are not Muslim? 

The answer was: 

This issue is based on differences of scholarly opinion concerning the interpretation of the aayah: 

“And tell the believing women to lower their gaze (from looking at forbidden things), and protect their private parts (from illegal sexual acts) and not to show off their adornment except only that which is apparent (like both eyes for necessity to see the way, or outer palms of hands or one eye or dress like veil, gloves, headcover, apron), and to draw their veils all over Juyoobihinna (i.e. their bodies, faces, necks and bosoms) and not to reveal their adornment except to their husbands, or their fathers, or their husband’s fathers, or their sons, or their husband’s sons, or their brothers or their brother’s sons, or their sister’s sons, or their women …” [al-Noor 24:31]

The scholars differed as to the meaning of the pronoun in the word nisaa’ihinna (their women). Some of them said that what it referred to was the gender, i.e., women in general. Some of them said that what it referred to was a specific type of women, i.e., believing women only. 

According to the first view, it is permissible for a woman to uncover her hair and face in front of a non-Muslim woman, and according to the second opinion it is not permissible. 

We are inclined towards the first view, which is more likely to be correct, because when a woman is with another woman there is no difference between a Muslim woman and a non-Muslim woman. This is the case so long as there is no fitnah or temptation. But if there is the fear of fitnah, such as the risk that she may describe the Muslim woman to her male relatives, then it is essential to take precautions against fitnah in that case, and she should not uncover any part of her body such as her legs or hair in front of another woman, regardless of whether she is Muslim or not. 

And Allah knows best. 

Fatawa al-Mar’ah al-Muslimah, 1/532, 533 

And Allah knows best.